Not sure what this error means, please see below for context.
It is asking us to login to the account, however the account was under an email address owned by an employee who has now left the company and email subsequently has been deleted.
If you can’t recover access to the account, then you’ll need to take control of DNS for the domain by changing the nameservers at the domain registrar to those for a new Cloudflare account or other DNS provider.
Recreate the mailbox or add it as an alias to an existing mailbox. Recover the account, then change the account to a shared service email and add additional users.